Th e soft annealing p ro cess sp h ero id izes an y exist in g h ard er flaky ele- m en t s o f t h e m icro st ru ct u re. W h ile t h e h ard n ess is ret ain ed , t h e flo w
st ress cf. Sect . 2.2.3
o f t h e m at erial is red u ced . Recrystallization annealing is ad van t ageo u s fo r au st en it ic st eels wit h a
lo w carbo n co n t en t , as wit h t h is h eat t reat m en t t h e lo west flo w st ress levels are o bt ain ed . Ho wever, if co arse grain o ccu rs as a resu lt o f lo cally
lo w levels o f t ru e st rain o r d efo rm at io n cf. Sect . 2.2.2
d u rin g recryst al- lizat io n an n ealin g, an d if t h is t yp e o f co arse grain is n o t p erm issible, a
n o rm alizat io n p ro cess sh o u ld be carried o u t . As a resu lt of two-fold recrystallization p erlite + ferrite = au sten ite
an even , fin e-grain ed an d fin e-lam ellar p erlite m icrostru ctu re is ach ieved th rou gh norm alization. Th is can be re-an n ealed if req u ired to create
grain ed p erlite soft-an n ealin g. A m arked d ifferen ce in m icrostru ctu re coarse an d fin e grain s can also resu lt d u rin g warm form in g as a resu lt of
locally d ifferin g coolin g con d ition s an d d egrees of tru e strain . Th ese d if- feren ces can cau se cracks d u rin g su bseq u en t cold sizin g op eration s
red u cin g, iron in g. Th is p roblem can be avoid ed by n orm al an n ealin g. Recovery annealing o ffers a lo w-co st alt ern at ive t o recryst allizat io n o r
so ft an n ealin g, p ro vid ed t h e d efo rm at io n in t h e su bseq u en t fo rm in g o p erat io n is m in im al, i. e. t h e level o f t ru e st rain is n o t t o o large.
6.3.3 Surface treatment
A d ifferen ce is d rawn bet ween abrad in g an d d ep o sit in g su rface t reat - m en t s. Abrad in g p ro cesses in clu d e ch em ical an d m ech an ical clean in g
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Fig. 6.3.2
Open and closed shearing knives and a closed shearing knife ground w ith elliptical relief
shearing clearance shearing clearance w edge
angle w edge angle
cutting edge
pressure surface
pressure surface flank surface
depth of relief grind cutting edge
elliptical relief grind
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
m et h o d s, d egreasin g an d d escalin g. Dep o sit in g p ro cesses in clu d e p h o s- p h at in g, so ap in g, m o lyco t in g an d t h e u se o f o ils. Th e p h o sp h at e layer
serves as a carrier layer wh ich facilit at es t h e ad h eren ce o f lu brican t s t o t h e su rface. So ap in g is u sed fo r sim p ler fo rm in g o p erat io n s, m o lyco t in g
fo r ext rem ely h igh d egrees o f d efo rm at io n cf. Sect . 2.2.2
an d fo r p art s su ch as gears. In t h e case o f h o rizo n t al m u lt i-st at io n p resses o p erat in g
wit h wire, o il is u sed fo r lu bricat io n an d co o lin g. In vert ical m u lt i- st at io n p resses, o il is u sed as a su p p lem en t ary lu brican t in ad d it io n t o
so ap m o lyco t e sp ray o r flo o d ap p licat io n cf. Sect . 6.8
. Cleaning, degreasing and descaling
Ch em ical clean in g an d degreasin g p rocesses are p erform ed eith er th rou gh im m ersion or sp rayin g of solven ts, or th rou gh th e con den sation of sol-
ven t vap or on th e workp ieces. Solven t typ es u sed in clu de organ ic solven ts h ydrocarbon s, p etroleu m eth er, p etroleu m , ch lorin ated h ydrocarbon s
trich loroeth ylen e an d p erch loroeth ylen e, an d water-solu ble clean in g agen ts su ch as acids, acid salin e solu tion s an d alkalin e solu tion s. Th e
m ech an ical m eth ods u sed in clu de san d blastin g or tu m blin g u sin g lim e, san d, steel sh ot an d oth er m aterials. Th e ch oice of clean in g m eth ods an d
agen ts dep en ds on th e typ e an d exten t of th e con tam in ation , th e req u ired degree of p u rity an d th e typ e, sh ap e an d q u an tity of th e item s to be
clean ed. En viron m en tally frien dly water-solu ble clean in g agen ts are fre- q u en tly u sed.
Ch em ical d escalin g p icklin g is p erform ed u sin g p icklin g bath s con - tain in g su lp h u ric acid or h yd roch loric acid solu tion s. Th e p icklin g agen ts
serve to release h yd rogen on th e su bstrate m etal, cau sin g th e oxid e layers to sp lit away. As th e absorp tion of h yd rogen in th e su bstrate m etal lead s
to em brittlem en t, as sh ort a p icklin g tim e as p ossible sh ou ld be u sed . Crack form ation d u rin g forgin g can , for exam p le, be th e resu lt of exces-
sive p icklin g treatm en t. San d blastin g an d tu m blin g are u sed for th e m ech an ical rem oval of th ick scale layers. Here, th e brittle scale layer is
p arted off from th e tou gh su bstrate m aterial th rou gh p artial ap p lication of force.
Phosphating Ph o sp h at in g is t aken t o m ean t h e gen erat io n o f co h esive cryst allin e
p h o sp h at e layers wh ich are firm ly ad h ered t o t h e su bst rat e m at erial. Fo r co ld fo rm in g p ro cesses, m ain ly zin c p h o sp h at e an d in so m e cases
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iron p h osp h ate layers are u sed . Ph osp h oric acid , zin c p h osp h ate an d oxid izin g agen ts are ap p lied in an aq u eou s solu tion . On im m ersion in
th e solu tion , th e m etal su rface is p ickled by th e p h osp h oric acid , d u rin g wh ich p rocess in solu ble zin c su lp h ate is sim u ltan eou sly p recip itated .
Th e ch em ical reaction d oes n ot com e to a stan d still u n til th e com p lete su rface is coated with zin c p h osp h ate crystals. Th e oxid izin g agen t
tran sform s th e d issolved m etal to a p h osp h ate with low solu bility wh ich is p recip itated in th e form of slu d ge an d h as to be rem oved p eriod ically.
In p ract ice, p h o sp h at e layers wit h a m ean t h ickn ess o f aro u n d 8 t o 15 m m are ap p lied , wh ich leave beh in d a sh in y co at in g aft er fo rm in g.
Th e p h o sp h at e layer can resist t em p erat u res u p t o 200 °C. O ver 200 °C, a p art ial t ran sfo rm at io n t akes p lace. At o ver 450 °C, co m p let e d eco m -
p o sit io n o f t h e p h o sp h at e layer t akes p lace. Dep en d in g o n t h e lu brican t u sed , h o wever, wo rkin g t em p erat u res o f u p t o 300 °C can be realized
d u rin g fo rgin g. Soaping and m olycoting
Soap in g takes p lace by im m ersion of th e p arts in an alkalin e soap solu - tion . Th e m ain com p on en t of th ese solu tion s is sod iu m stearate. A p art
of th e zin c p h osp h ate layer reacts with th e sod iu m soap to form a water- in solu ble zin c soap layer. Th e zin c soap is ad d ition ally coated with th e
sod iu m soap . Th is com bin ation of layers h as a very low sh ear stren gth an d th u s red u ces th e effective coefficien ts of friction cf.
Sect. 4.2.3 .
Soap in g sh ou ld n ot take lon ger th an 5 m in , as oth erwise th e en tire p h os- p h ate layer will be u sed u p an d n o lon ger p rovid e an ad h esive su bstrate.
To m ake su re t h at t h e so ap co at in g o ffers go o d lu bricat io n p ro p ert ies it is n ecessary t o d ry t h e co at ed billet s co m p let ely. A wait in g p erio d o f
o n e d ay p revio u s t o p ro cessin g o f t h e t reat ed blan ks is u su ally reco m - m en d ed . Th e so ap s are t em p erat u re-resist an t t o ap p ro x. 250 °C, abo ve
wh ich ch em ical t ran sfo rm at io n p ro cesses t ake p lace. Excessive so ap in g resu lt s in so ap d ep o sit s bu ild in g u p o n t h e d ies.
Mo lybd en u m d isu lp h id e is u sed u n d er d ifficu lt fo rm in g co n d it io n s. Co m p ared t o so ap lu brican t s, m o lybd en u m d isu lp h id e cau ses great er
frict io n lo sses wh ich m ean t h at larger p ress an d eject o r fo rces h ave t o be t aken in t o co n sid erat io n . Ho wever, it is cap able o f wit h st an d in g far
great er levels o f st ress, i. e. t h e lu brican t film o n ly breaks away u n d er ext rem ely h igh d egrees o f su rface p ressu re, an d p erm it s co n sid erable
su rface exp an sio n o f t h e wo rkp iece. Th erefo re, m o lybd en u m d isu l-
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p h id e is u sed wh en large d efo rm at io n s, i. e. large t ru e st rain s, are p re- sen t . In ad d it io n , m o lybd en u m d isu lp h id e creat es fewer p ro blem s as
regard s t h e accu m u lat io n o f su rp lu s lu brican t in t h e d ies. Th e u p p er ap p licat io n t em p erat u re lim it is aro u n d 350 °C. At h igh er t em p erat u res,
m o lybd en u m t risu lp h id e is creat ed , wh ich is in effect ive as a lu brican t . Mo lybd en u m d isu lp h id e is also ap p lied in an im m ersio n bat h , an d
in so m e cases sp rayed o r ap p lied by t u m blin g. Th e co at ed billet s sh o u ld be st o red fo r o n e d ay p rio r t o p ro cessin g. A st o rage p erio d o f o n e week
sh o u ld n o t be exceed ed . As a ro u gh gu id elin e, t h e o verall seq u en ce o f su rface t reat m en t wit h
t h e resp ect ive fu n ct io n s o f d ifferen t bat h s, bat h co m p o sit io n , t im es an d t em p erat u res is su m m arized belo w:
1. Degreasin g: Ch em ical d egreasin g in a 5 so d iu m h yd ro x-
id e so lu t io n at 80 t o 95 °C fo r aro u n d 5 t o 10 m in .
2. Cold rin sin g: Rem oval of sod iu m solu tion resid u es, cold rin s-
in g in ru n n in g water for ap p rox. 2 m in water su rface m u st always be clean .
3. Picklin g: Picklin g in 12 su lp h u ric acid o r 18 h yd ro -
ch lo ric acid at ap p ro x. 50 t o 65 °C, d u rat io n ap p ro x. 10 t o 18 m in acid co n t en t is co rrect if
t h e p ickle effervesces. 4. Co ld rin sin g:
Rem o val o f acid resid u es, co ld risin g in ru n - n in g wat er fo r ap p ro x. 2 m in .
5. Ho t rin sin g: Ho t rin sin g t o p re-h eat t h e p art s – in flu en ces
t h e p h o sp h at e layer t h ickn ess, 70 – 90 °C, fo r aro u n d 2 t o 3 m in at lo wer t em p erat u res, a
lo wer layer t h ickn ess resu lt s. 6. Ph o sp h at in g:
Main ly u sin g zin c p h o sp h at e, fo r ru st p ro o f st eels wit h iro n o xo lat e, 70 t o 80 °C fo r aro u n d
8 t o 10 m in p h o sp h at in g co m p rises an in it ial p icklin g react io n an d a su bseq u en t layer
fo rm in g react io n , layer t h ickn ess ap p ro x. 8 t o 15 m m .
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7. Co ld rin sin g: Co ld rin sin g in ru n n in g wat er fo r ap p ro x.
2 m in . 8. Hot rin sin g:
Hot rin sin g to p re-h eat th e p arts at arou n d 60 °C, d u ration ap p rox. 3 m in .
9. So ap in g: So ap in g at 60 t o 85 °C fo r ap p ro x. 5 m in lo w-
er t em p erat u res resu lt in a t h icker, lo wer t em - p erat u res in a t h in n er layer.
10. Dryin g: Dryin g wit h h o t air at ap p ro x. 120 °C fo r
ap p ro x. 10 m in . In t h e case o f n o n -ferro u s m et als, it is n o t n ecessary t o u se lu brican t car-
ryin g layers. Clean in g an d lu bricat io n is all t h at is req u ired . In t h e case o f co p p er, wh ere lo w d egrees o f d efo rm at io n are req u ired , lu bricat io n
can be elim in at ed alt o get h er. Oil as a lubricant
W h ile so ap in g an d m o lyco t in g p ro cesses are m ain ly u sed wit h billet s, in m u lt i-st at io n p resses, wh en fo rgin g fro m wire, m in eral-based o ils
wit h ad d it ives EP ad d it ives are gen erally u sed t o im p ro ve lu bricat io n p ro p ert ies. Th e o il also ad h eres bet t er t o t h e ro u gh p h o sp h at e layer.
Wit h t h is t yp e o f p ro d u ct io n , t h e lu bricat io n o f t h e flat face su rfaces p resen t s a p ro blem becau se t h ese are n o t co at ed wit h a p h o sp h at e lay-
er d u e t o t h e sh earin g p ro cess. Th e o il is sp rayed o r flo o d ed in t o t h e d ies o r o n t o t h e wo rkp ieces.
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6 Solid forming Forging
6.4 Formed part and process plan
